Conductive roll

ABSTRACT

A conductive roll including a conductive shaft, a conductive rubber foam layer formed on an outer circumferential surface of the conductive shaft, and a semi-conductive solid rubber layer formed on an outer circumferential surface of the conductive rubber foam layer. The conductive roll is produced by forming a cylindrical laminar body which gives the conductive rubber foam layer and semi-conductive solid rubber layer, and simultaneously vulcanizing and foaming the laminar body in a mold in which the conductive shaft is placed in position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrically conductive roll whichis particularly favorably used as a charging roll or an image developingroll for an electrophotographic copying machine, printer or the like.This invention is also concerned with a method of producing such aconductive roll as described above.

2. Discussion of Related Art

An electrically conductive roll, such as a charging roll or an imagedeveloping roll used in an electrophotographic copying machine,facsimile machine or the like, has been desired to meet the followingrequirements: (1) the roll can be held in good contact with aphotosensitive drum, (2) the photosensitive drum is not contaminated bya softener which oozes or comes out onto the outer surface of the roll,(3) the electric resistance of the roll is suitably controlled, and (4)the roll has relatively small tackiness and is less likely to stick tothe photosensitive drum.

To fulfill all the above requirements, an electrically conductive rollas shown in FIG. 6 has been proposed by the assignee of the presentapplication in laid-open Publication No. 2-311868 of unexamined JapanesePatent Application. As is apparent from FIG. 6, the conductive roll 42has an electrically conductive shaft 44, and a base layer 46 which isformed on the outer circumferential surface of the shaft 44 and is madeof an electrically conductive, elastic material. On the outercircumferential surface of the base layer 46, there are formed bycoating a softener-preventing layer 48 (which will be described later),a resistance adjusting layer 50 formed of a particular semi-conductiverubber, and a protective layer 52 formed of a semi-conductive resin, inthe order of description. The preventive layer 48 is formed of asuitable conductive resin which contains an electron-conductivematerial.

In the conductive roll 42 constructed as described above, the conductiveelastic material for the base layer 46 contains a suitable softenerwhich serves to lower the hardness of the base layer 46, and thusadvantageously enhance contact between the roll 42 and a photosensitivedrum (not shown). The softener-preventing layer 48 formed on the outersurface, of the base layer 46 serves to prevent the softener containedin the base layer 46 from oozing or migrating onto the outer surface ofthe roll 42, thereby to effectively avoid contamination of thephotosensitive drum. The above-indicated resistance adjusting layer 50is formed of a conductive material having a volume resistivity which isheld in a suitable range such that the electrical resistance of the rollis desirably controlled within an appropriate range. Further, theprotective layer 42 as the outermost layer of the roll 42 effectivelyprevents the roll 42 from sticking to the photosensitive drum which theroll 42 is to be pressed against and held in contact with. Thus, in theconductive roll 42 as disclosed in the above-identified publication, thefour individual layers 46, 48, 50, 52 formed of different materials arestacked or laminated integrally into a tube formed on the outercircumferential surface of the shaft 44, so that the resulting roll 42satisfactorily meets all the requirements (1) through (4) as describedabove.

As well known in the art, when a voltage is applied to a charging rollas one type of the conductive roll while the roll is being pressedagainst and held in contact with a photosensitive drum, thephotosensitive drum vibrates due to force which acts between thecharging roll and the photosensitive drum upon changes in the frequencyof AC fields. When the charging roll is used in a high-speed copyingmachine, printer or the like which performs its copying or printingoperation at a high speed, in particular, the vibration of the drum isincreased so much as to cause undesirable noise. Therefore, theconductive roll, particularly the charging roll, is required to have anexcellent vibration absorbing characteristic, as well as theabove-described characteristics. The conductive roll having all therequired characteristics is capable of absorbing the vibration of thephotosensitive drum, and easily and effectively avoiding the noise.

To deal with the vibration which takes place between the conductive rolland the photosensitive roll and causes noises, the above-identifiedpublication proposes that a relatively large amount of softener becontained in the base layer 46 to reduce the hardness of the layer 46,so as to effectively prevent the vibration as described above. However,the thus obtained conductive roll 42 inevitably suffers from anincreased amount of the softener which oozes out of the roll 42 due tothe large content of the softener in the base layer 46.

The oozing of the softener may be prevented to some extent by increasingthe thickness of the softener-preventing layer 48 of the conductive roll42. However, the increase in the thickness of the softener-preventinglayer 48 causes other problems during the process of forming the layer48 by coating, for example, sagging of a coating liquid which forms thesoftener-preventing layer 48. Thus, the conductive roll 42 as disclosedin the above publication still has some room for improvement in terms ofits vibration absorbing characteristic.

The conductive roll 42 exhibits the above-described excellentcharacteristics due to provision of the softener-preventing layer 48,resistance adjusting layer 50 and protective layer 52 on the outersurface of the base layer 46 formed on the outer surface of the shaft44. Since these three layers 48, 50, 52 are formed by coating on thebase layer 46, the conductive roll 42 suffers from various problems interms of its product capability or performance and the process ofmanufacturing the roll 42.

More specifically, in order to achieve the uniform thickness of each ofthe coating layers 48, 50, 52 of the conductive roll 42 in thecircumferential direction thereof, the coating operation is effectedwhile a base roll consisting of the shaft 44 and the base layer 46 isbeing supported so as to stand upright. During this coating operation,sagging of a coating liquid which gives each of the layers 48, 50, 52inevitably takes place, whereby the thickness of each coating layer 48,50, 52 is undesirably increased in the direction from one axial endthereof toward the other end. If the coating liquid sags excessively, aclearance or gap appears between the obtained conductive roll 42 and thephotosensitive drum, resulting in uneven charging of the drum. Further,corona discharge may take place exclusively at the clearance between theroll 42 and the photosensitive drum, and a portion of the photosensitivedrum which corresponds to the clearance may wear faster than the otherportions in a long period of use, whereby the electrical resistance ofthe photosensitive drum is lowered. As a result, an abnormal dischargemay take place upon application of a low voltage. In an extreme case,the image reproduced by the conductive roll has a low copy quality, thatis, lines undesirably appear as a part of the reproduced image in thetransverse direction of the copy sheet.

In producing this conductive roll 42, a solvent is used in theabove-described coating operation for dissolving a suitable rubbermaterial or resin material. The use of the solvent requires considerableconcern for safety, and also causes bubbles and cissing or crawling toappear on the coating layers 48, 50, 52. To enhance yield of theconductive roll 42, the temperature, humidity, gas volume or quantityfor drying the coating layers 48, 50, 52 need to be considerablyaccurately controlled. Moreover, there is a limit to the material usedfor each of the coating layers 48, 50, 52, which is selected dependingupon a specific kind of the solvent used for dissolving the material.

Where a granular electron-conductive material is added to the materialfor any of the coating layers 48, 50, 52 of the conductive roll 42, thecoating liquid including the conductive material must be constantlystirred during the coating operation, so as to reduce a variation of theelectrical resistance in the coating layer 48, 50, 52. It necessitatesbulky equipment which is capable of effecting the coating while stirringthe coating liquid.

In the conductive roll 42 as disclosed in the above-identifiedpublication, the resistance adjusting layer 50 is required to have arelatively large thickness of about 100-200 μm, so as to assure anincreased resistance to leak (leakage of electric current). Since such aresistance adjusting layer 50 cannot be formed by one coating cyclewithout causing sagging of the coating liquid as described above, thecoating cycle must be repeated many times to gradually increase thethickness of the resistance adjusting layer 50. Further, the viscosityof the coating liquid must be also controlled so as to assure highuniformity in the thickness of the layer 50. These requirementseventually make the process of forming the three coating layersincluding the resistance adjusting layer 50 extremely cumbersome and lowefficient.

Moreover, the conductive roll 42 has C-shaped or R-shaped axiallyopposite ends so as to avoid spark discharge at the axial end facesthereof. Therefore, the thickness of each of the coating layers 48, 50,52 is locally reduced due to the surface tension of the coating liquid,at a point of inflection formed at the boundary of the axial endportions of the roll 42 and its axially middle portion. Accordingly, theresultant roll 42 inevitably includes a portion having a relativelysmall thickness even if the thickness of the resistance adjusting layer50 is accurately controlled by the cumbersome process as describedabove. This results in high possibility of leakage of electric currentdue to discharge breakdown occurring at the small-thickness portion ofthe roll 42.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide aconductive roll which has significantly reduced hardness without relyingmuch on a softener, assuring an effectively improved vibration absorbingcharacteristic.

It is a second object of the invention to provide a method of producingsuch a conductive roll with ease and safety, without suffering from theabove-described problems encountered in the conventional method. Theconductive roll thus produced exhibits high and stable operatingcapability or performance when installed in a printer or the like.

It is a third object of the invention to provide a method of producing aroll with ease and safety, which roll exhibits excellent operatingcapability.

The above first object may be accomplished according to a first aspectof the present invention, which provides a conductive roll comprising aconductive shaft, a conductive rubber foam layer formed on an outercircumferential surface of the conductive shaft, and a semi-conductivesolid rubber layer formed on an outer circumferential surface of theconductive rubber foam layer.

The conductive roll constructed as described above has significantlyreduced hardness without using a large amount of a softener, assuring agood contact with a photosensitive drum. Thus, the conductive roll hasan effectively improved vibration absorbing characteristic, and does notsuffer from noise.

In one preferred form of the first aspect of the invention, theconductive roll further includes a semi-conductive thermoplastic resinlayer formed by coating on an outer surface of the semi-conductive solidrubber layer. This semi-conductive thermoplastic resin layer is madeprincipally of a composition which contains a thermoplastic resincontaining conductive particles dispersed therein, or the thermoplasticresin further containing a crosslinking agent.

In another preferred form of the invention, the conductive rubber foamlayer contains conductive particles and has a volume resistivity of notgreater than 10⁶ Ωcm.

In a further preferred form of the invention, the semi-conductive solidrubber layer contains an ion-conductive material, and a volumeresistivity of 10⁵ Ωcm-10¹⁰ Ωcm.

The above-identified first object may be accomplished according to asecond aspect of the present invention, which provides a conductive rollcomprising a conductive shaft, a semi-conductive rubber foam layerformed on an outer circumferential surface of the conductive shaft, aconductive solid rubber layer formed on an outer circumferential surfaceof the semi-conductive rubber foam layer, and a semi-conductive solidrubber layer formed on an outer circumferential surface of theconductive solid rubber layer.

In one preferred form of the second aspect of the invention, theconductive roll further includes a semi-conductive thermoplastic resinlayer formed by coating on an outer surface of the semi-conductive solidrubber layer. This semi-conductive thermoplastic resin layer is madeprincipally of a composition which contains a thermoplastic resincontaining conductive particles dispersed therein, or the thermoplasticresin further containing a crosslinking agent.

In another preferred form of the invention, the semi-conductive rubberfoam layer contains an ion-conductive material, and a volume resistivityof 10⁶ Ωcm-10⁹ Ωcm.

In a further preferred form of the invention, the conductive solidrubber layer contains conductive particles, and has a volume resistivityof not greater than 10⁶ Ωcm.

In a still further preferred form of the invention, the semi-conductiverubber layer contains an ion-conductive material, and a volumeresistivity of 10⁶ Ωcm-10¹⁰ Ωcm.

The above-indicated second object of the invention may be accomplishedaccording to a third aspect of the present invention, which provides amethod of producing a conductive roll including a conductive shaft, aconductive rubber foam layer formed on an outer circumferential surfaceof the conductive shaft, and a semi-conductive solid rubber layer formedon an outer circumferential surface of the conductive rubber foam layer,comprising the steps of: (a) positioning the conductive shaft in acavity of a cylindrical mold, (b) forming a cylindrical laminar bodywhich has an inner layer consisting of an unvulcanized, unfoamed rubberlayer that gives the conductive rubber foam layer, and an outer layerconsisting of an unvulcanized, non-foaming rubber layer that gives thesemi-conductive solid rubber layer, the laminar body having an insidediameter which is larger than an outside diameter of the conductiveshaft, and an outside diameter which is smaller than an inside diameterof the cavity of the cylindrical mold; (c) disposing the laminar bodycoaxially with the conductive shaft in the cavity of the cylindricalmold; and (d) vulcanizing the unvulcanized, unfoamed rubber layer andthe unvulcanized, non-foaming rubber layer and foaming the unvulcanized,unfoamed rubber layer at the same time.

According to the above-described method, the conductive roll having highand stable operating capability can be easily and safely producedwithout causing the conventional problems as described above.

Preferably, the cylindrical laminar body is formed by concurrentlyextruding a conductive rubber composition and a semi-conductive rubbercomposition so as to form the unvulcanized, unfoamed rubber layer andthe unvulcanized, non-foaming rubber layer, respectively.

The above second object may be accomplished according to a fourth aspectof the present invention, which provides a method of producing aconductive roll including a conductive shaft, a semi-conductive rubberfoam layer formed on an outer circumferential surface of the conductiveshaft, a conductive solid rubber layer formed on an outercircumferential surface of the semi-conductive rubber foam layer, and asemi-conductive solid rubber layer formed on an outer circumferentialsurface of the conductive solid rubber layer, comprising the steps of:(a) positioning the conductive shaft in a cavity of a cylindrical mold;(b) forming a cylindrical laminar body which has an inner layerconsisting of an unvulcanized, unfoamed rubber layer that gives thesemi-conductive rubber foam layer, an intermediate layer consisting ofan unvulcanized, non-foaming conductive rubber layer that gives theconductive solid rubber layer, and an outer layer consisting of anunvulcanized, non-foaming semi-conductive rubber layer that gives thesemi-conductive solid rubber layer, the laminar body having an insidediameter which is larger than an outside diameter of the conductiveshaft, and an outside diameter which is smaller than an inside diameterof the cavity of the cylindrical mold; (c) disposing the laminar bodycoaxially with the conductive shaft in the cavity of the cylindricalmold; and (d) vulcanizing the unvulcanized, unfoamed rubber layer, theunvulcanized, non-foaming conductive ribber layer and the unvulcanized,non-foaming semi-conductive rubber layer of the cylindrical laminar bodyand foaming the unvulcanized, unfoamed rubber layer at the same time.

Preferably, the cylindrical laminar body is formed by concurrentlyextruding a semi-conductive rubber composition which gives theunvulcanized, unfoamed rubber layer, a conductive rubber compositionwhich gives the unvulcanized, non-foaming conductive rubber layer, and asemi-conductive composition which gives the unvulcanized, non-foamingsemi-conductive rubber layer.

The above-indicated third object may also be attained according to afifth aspect of the present invention, which provides a method ofproducing a roll including a shaft, an elastic foam layer formed on anouter circumferential surface of the shaft, at least one elastic solidlayer formed on an outer circumferential surface of the elastic foamlayer, comprising the steps of: (a) positioning the shaft in a cavity ofa cylindrical mold; (b) forming a cylindrical laminar body which has aninner layer consisting of an unvulcanized, unfoamed elastic layer thatgives the elastic foam layer, and an outer layer consisting of at leastone unvulcanized, non-foaming elastic layer that gives the elastic solidlayer or layers, the laminar body having an inside diameter which islarger than an outside diameter of the shaft, and an outside diameterwhich is smaller than n inside diameter of the cavity of the cylindricalmold; (c) disposing the laminar body coaxially with the shaft; and (d)vulcanizing the unvulcanized, unfoamed elastic layer and at least oneunvulcanized, non-foaming elastic layer, and foaming the unvulcanized,unfoamed elastic layer at the same time.

Preferably, the cylindrical laminar body is formed by concurrentlyextruding a composition which gives the unvulcanized, unfoamed elasticlayer, and at least one composition which gives the above-indicated atleast unvulcanized, non-foaming elastic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and optional objects, features and advantages of the presentinvention will be better understood by reading the following detaileddescription of presently preferred embodiments of the invention, whenconsidered in connection with the accompanying drawings, in which:

FIG. 1 is an axial cross-sectional view showing one embodiment of aconductive roll of the present invention;

FIG. 2 is a view illustrating one step of the process of producing theconductive roll of FIG. 1, wherein a laminar body having two rubberlayers is formed by extrusion;

FIG. 3 is a view illustrating the same step of the process of producingthe conductive roll of FIG. 1, wherein the two layers of the laminarbody are extruded at the same time;

FIG. 4 is a view illustrating another step of the process of producingthe conductive roll of FIG. 1, wherein the laminar body is located in acylindrical metallic mold for subsequent vulcanizing and foamingoperation;

FIG. 5 is a view corresponding to that of FIG. 1, showing anotherembodiment of the conductive roll of the present invention; and

FIG. 6 is a view corresponding to that of FIG. 1, showing a conventionalconductive roll.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, a conductive roll 10 as one preferredembodiment of the present invention includes an electrically conductiveshaft 12, an electrically conductive, rubber foam layer 14 formed on theouter circumferential surface of the shaft 12, and a semi-conductivesolid rubber layer 16 formed on the outer circumferential surface of thefoam layer 14.

The conductive shaft 12 located at the axis of the conductive roll 10has a columnar shape, and is formed of a known material, such as metalsincluding iron, aluminum and various kinds of stainless steel, andelectrically conductive synthetic resins. When the conductive shaft 12is formed of iron, the outer circumferential surface of the shaft 12 isplated with a suitable material as needed.

The conductive rubber foam layer 14 formed on the outer surface of theconductive shaft 12 constitutes a base layer of the conductive roll 10.This foam layer 14 is formed of a conductive rubber composition whichprincipally consists of a mixture of a suitable rubber material,conductive particles and a foaming agent. More specifically, the rubbermaterial may consist solely of natural rubber, or a synthetic rubberselected from styrene-butadiene rubber, ethylene-propylene rubber,butadiene rubber, chloroprene rubber, nitrile rubber, butyl rubber,Hypalon, acrylic rubber, fluorinated rubber, silicone rubber andisoprene rubber, or may be a mixture of two or more of theabove-indicated rubbers. The selected rubber material is mixed with theconductive particles, such as carbon black, graphite, metal powder, orconductive metal oxide (e.g., stannic oxide, titanium oxide or zincoxide), and further with an organic foaming agent, such as dinitrosopentamethylene tetramine, azodicarbonamide, azobisisobutyronitrile,4,4'-hydroxybisbenzene, p-toluene sulfonyl-hydrazone, or an inorganicfoaming agent such as sodium bicarbonate. The thus prepared conductiverubber composition may further contain as needed suitable amounts ofvarious known compounding agents or additives, such as a vulcanizingagent, vulcanizing aid, softener, filler and processing agent.

The conductive rubber foam layer 14 prepared from the above-describedrubber composition containing the conductive particles is provided witha desired electrical conductivity. Further, the above rubber compositioncontaining the foaming agent provides a foam body having an effectivelyreduced hardness as the rubber foam layer 14, and thus eliminates a needto use a large amount of softener for achieving sufficiently lowhardness.

The contents of the foaming agent and the conductive particles in therubber composition for the conductive rubber foam layer 14 are suitablydetermined depending upon desired hardness and electricalcharacteristics of the conductive roll 10. In this embodiment, theamount of the conductive particles is desirably determined so that thevolume resistivity of the rubber foam layer 14 is not greater than 10⁶Ωcm, so as to achieve high uniformity of the electrical characteristicsof the conductive rubber layer 14.

The semi-conductive rubber layer 16 formed integrally on the outersurface of the above-described conductive rubber foam layer 14 is formedof a semi-conductive rubber composition which principally consists of amixture of a suitable rubber material and conductive particles (anelectron-conductive agent), or a mixture of a suitable rubber materialand an ion-conductive material.

More specifically, the rubber material to be mixed with the conductiveparticles to provide the semi-conductive rubber composition may consistsolely of natural rubber, or a synthetic rubber selected fromstyrene-butadiene rubber, ethylene-propylene rubber, butadiene rubber,chloroprene rubber, nitrile rubber, butyl rubber, Hypalon, acrylicrubber, fluorinated rubber, silicone rubber or isoprene rubber, or maybe a mixture of two or more of the above-indicated rubbers. The selectedrubber material is mixed with the conductive particles, such as carbonblack, graphite, or metal powder. On the other hand, the rubber materialto be mixed with the ion-conductive material may consist solely of asynthetic rubber selected from epichlorohydrin-ethylene oxide rubber,nitrile rubber, propylene oxide rubber, polyether-type urethane rubberand acrylic rubber, or may be a mixture of two or more of theabove-indicated rubbers. The selected rubber material is mixed with theion-conductive material which is selected from among quaternary ammoniumsalts, such as trimethyloctadecyl ammonium perchlorate andbenzyltrimethyl ammonium chloride, and structural charge specificanions, such as lithium perchlorate and potassium perchlorate. It is tobe understood that the semi-conductive rubber composition for the rubberlayer 16 may further contain suitable amounts of various knowncompounding agents and additives, as needed.

The semi-conductive rubber layer 16 prepared from the abovesemi-conductive rubber composition exhibits excellent flexibility due tothe use of the rubber material as a major component. The semi-conductiverubber layer 16 also contains the conductive particles or ion-conductivematerial for giving the layer 16 semi-conductivity, and thus effectivelyfunctions as a resistance adjusting layer of the conductive roll 10. Theamount of the conductive particles or ion-conductive material containedin the rubber composition is desirable controlled so that thesemi-conductive rubber layer 16 has a volume resistivity in the range of10⁶ -10¹⁰ Ωcm. Consequently, the semi-conductive rubber layer 16provides a uniform electrical resistance over its entire surface area,and thus exhibits enhances electrical characteristics required for thelayer 16 to serve as the resistance adjusting layer. Where thesemi-conductive rubber layer 16 contains the ion-conductive materialwhich makes the layer 16 ion-conductive, the obtained conductive roll 10does not suffer from an increase in the hardness thereof due to the useof the conductive particles, and thus advantageously exhibits relativelylow hardness.

Thus, the electrical resistance of the conductive roll 10 according tothe present invention is controlled to within an appropriate range dueto the provision of the semi-conductive rubber layer 16 as theresistance adjusting layer, whereby the conductive roll 10 exhibitsexcellent voltage resistance or leak resistance. Further, the conductiverubber foam layer 14 which consists of a rubber foam body provides thebase layer which bears thereon the resistance adjusting layer, and thebase layer exhibits a significantly reduced hardness without relyingmuch on the softener. Thus, the amount of the softener contained in thebase layer can be considerably reduced, as compared with that of theconventional roll, and the thus reduced softener is not likely to oozeor come out of the base layer. Consequently, the present conductive roll10 can be held in good contact with the photosensitive drum withoutsuffering from problems, such as contamination of the drum or stickingof the roll to the drum, which would be otherwise caused by oozing ofthe softener. The present conductive roll 10 also has a significantlyimproved vibration absorbing characteristic, and does not suffer fromnoise during its operation.

Since both the base layer and the resistance adjusting layer in the formof the conductive rubber foam layer 14 and the semi-conductive solidrubber layer 16 are formed of rubber compositions which contain a rubbermaterial as a major component, the base and resistance adjusting layerscan be concurrently formed by vulcanizing those layers at the same time.The formation of these two layers in this manner does not involve theconventional problem that the semi-conductive rubber layer formed on theouter surface of the conductive rubber foam layer is broken or damageddue to the volume expansion of the foam layer upon foaming of thematerial which gives the foam layer.

Preferably, the conductive roll 10 according to the present inventionfurther includes a semi-conductive thermoplastic resin layer 18 whichserves as a protective layer, as shown in FIG. 1. This thermoplasticresin layer 18 is formed by coating on the outer circumferential surfaceof the semi-conductive rubber layer 16, and is made of a thermoplasticresin in which conductive particles are dispersed, or a compositionprepared by mixing the thermoplastic resin with a crosslinking agent.

More specifically, the semi-conductive thermoplastic resin layer 18 isformed of a resin composition that is prepared by mixing conductiveparticles by dispersion with a thermoplastic resin, such as 8-nylon,butyral, urethane, copolymer of 4-ethylene fluoride, vinylidenefluoride, or copolymer of 4-ethylene fluoride and vinylidene fluorideand hexafluoropropylene. The conductive particles may be selected fromcarbon black and various conductive metal oxides, such as stannic oxide,titanium oxide and zinc oxide. This resin composition may furthercontain a known crosslinking agent, such as that containing isocyanate.It is to be understood that various known compounding agents may beadded as needed to the resin composition thus prepared.

In the conductive roll 10 as described above, the semi-conductivethermoplastic resin layer 18 formed of the above-described materialserves as a protective layer which effectively prevents the componentsof the semi-conductive rubber layer 16 formed inside the resin layer 18from coming out onto the roll surface and contaminate the photosensitivedrum. This thermoplastic resin layer 18 also prevents the conductiveroll 10 from sticking to the photosensitive drum when the roll 10 isheld in pressed contact with the drum for a long period of time. Theconductive thermoplastic resin layer 18 is formed by coating with aconsiderably small thickness of several microns to several tens ofmicrons, without increasing the hardness of the roll 10. When thesemi-conductive thermoplastic resin layer 18 is formed of the resincomposition containing a particular crosslinking agent, the resultantconductive roll 10 is further less likely to stick to the photosensitivedrum.

The conductive roll 10 having the above-described excellentcharacteristics will be produced by the following method.

Initially, a cylindrical laminar body is formed by using theabove-described conductive rubber composition for the conductive rubberfoam layer 14, and the semi-conductive rubber composition for thesemi-conductive rubber layer 16. This cylindrical laminar body has anunvulcanized, unfoamed rubber layer (which will be foamed later) as aninner layer that gives the conductive rubber foam layer 14, and anunvulcanized, non-foaming rubber layer as an outer layer which gives thesemi-conductive solid rubber layer 16.

The cylindrical laminar body is desirably formed by first forming byextrusion a two-layered elongate tube formed of the above conductiverubber composition and semi-conductive rubber composition, and thencutting the tube into segment each having a predetermined length. Whilethe cylindrical laminar body may be formed by other methods, the abovemethod is advantageous in that a large number of cylindrical laminarbodies can be continuously formed one by one, and each layer of theobtained laminar body has considerably uniform dimensions as measured inthe axial and circumferential directions. The cylindrical laminar bodymay be also formed by forming separate tubes one of which consistssolely of the unvulcanized, unfoamed rubber layer, the other consistingsolely of the unvulcanized, non-foaming rubber layer, and then insertingthe above-indicated one tube into the other tube.

Referring to FIG. 2, when the cylindrical laminar body is formed byextrusion as described above, the unvulcanized, unfoamed rubber layer 20is initially formed by extrusion into a tubular shape with an extruder(not shown), and the unvulcanized, non-foaming rubber layer 22 is thenextruded by an extruder 19 so as to cover the outer circumferentialsurface of the tube-like rubber layer 20. In this manner, a continuouslaminar tube 23 is formed which has an inner layer that consists of theunvulcanized, unfoamed rubber layer 20, and an outer layer that consistsof the unvulcanized, non-foaming rubber layer 22. Then, the laminar tube23 is cut into a plurality of laminar bodies 24 each having apredetermined length. According to another method as shown in FIG. 3,the conductive rubber composition and the semi-conductive rubbercomposition are concurrently passed through a single extruder 21 whichis capable of extruding two tubes at a time, so as to form the laminartube 23 constructed as described above, which is then cut into thelaminar bodies 24. The latter method is advantageous in that the laminartube 23 which gives the laminar bodies 24 can be formed in one processstep wherein two tubes are extruded at a time, thus effectivelysimplifying the process of forming the laminar bodies 24.

The cylindrical laminar body 24 formed by either of the above methodshas an inside diameter which larger than the outside diameter of theconductive shaft 12, and an outside diameter which is smaller than theinside diameter of a molding cavity of a cylindrical metallic mold 26(FIG. 4.) that will be used for the following vulcanizing and foamingoperations. The thus dimensioned laminar body 24 can be placed inposition within the cavity of the cylindrical mold 26, as shown in FIG.4, such that the conductive shaft 12 is located in an inner bore of thelaminar body 24. The thickness of each of the unvulcanized, unfoamedrubber layer 20 and the unvulcanized rubber layer 22 is suitablydetermined so that the conductive rubber foam layer 14 andsemi-conductive rubber layer 16 provided by these layers 20, 22 havedesired thickness values which are determined depending upon theparticular application of the conductive roll 10, for example.

After or before forming the laminar body 24 in the manner as describedabove, the conductive shaft 12 is positioned at approximately the centerof the cavity of the cylindrical metallic mold 26. In this step, asuitable conductive adhesive may be applied as needed to the outercircumferential surface of the conductive shaft 12.

In the next step, the laminar body 24 is positioned within the cavity ofthe cylindrical metal mold 26, as shown in FIG. 4, such that the laminarbody 24 is coaxial or concentric with the conductive shaft 12, and suchthat suitable clearances are formed between the laminar body 24 and theshaft 12 and between the laminar body 24 and a cylindrical portion ofthe metal mold 26.

Thereafter, the vulcanizing and foaming operations are concurrentlyeffected by heating, for example, to vulcanize and foam theunvulcanized, unfoamed rubber layer 20 and vulcanize the unvulcanizedrubber layer 22, thereby to provide the conductive rubber foam layer 14and the semi-conductive rubber layer 16, respectively. Since the volumeof the unvulcanized, unfoamed rubber layer 20 expands upon foamingthereof, the conductive rubber foam layer 14 is formed integrally withthe semi-conductive rubber layer 16 and the conductive shaft 12, so asto provide a desired conductive roll 10.

In the above-described method of producing the conductive roll 10, theconductive rubber foam layer 14 as the base layer and thesemi-conductive rubber layer 16 as the resistance adjusting layer areformed without effecting any coating operation, thus eliminating theconventional problems, such as sagging of a coating liquid during thecoating operation, or reduced thickness in the end portions of the roll10. Accordingly, the conductive rubber foam layer 14 and semi-conductiverubber layer 16 assure improved uniformity in the thickness thereof,which leads to improved contact between the roll 10 and thephotosensitive drum, and improved resistance to leak (leakage ofelectric current). In addition, the conductive roll 10 is free fromabnormal discharge which may cause defects in reproduced images, andthus assure high and stable operating capability when installed in acopying machine, printer or the like.

Further, the above-described method according to the present inventiondoes not employ any solvent for a coating liquid or slurry, andtherefore assures improved safety in the manufacturing process, andincreased freedom of choice of the materials for the conductive rubberfoam layer 14 and the semi-conductive rubber layer 16. Moreover, thecumbersome drying step and control of the viscosity for the coatingliquid can also be eliminated, resulting in simplified equipment forforming the rubber layers 14, 16.

According to the method of the present invention, therefore, theconductive roll 10 having the above-described excellent characteristicscan be easily and safely produced.

Further, according to the method of the present invention, the surfaceof the obtained conductive roll 10 is formed so as to conform to theinner circumferential surface of the cylindrical metal mold 26, due tothe volume expansion of the unvulcanized, unfoamed rubber layer 20 uponfoaming thereof. Accordingly, the smoothness of the surface of theconductive roll 10 can be advantageously enhanced by increasing theflatness or smoothness of the inner circumferential surface of the metalmold 26, thus eliminating the necessity of grinding the roll surfaceafter the vulcanizing and foaming operations. This eventually reducesthe number of steps required for producing the present conductive roll10.

To form the semi-conductive thermoplastic resin layer 18 on the outersurface of the semi-conductive rubber layer 16 of the conductive roll 10as described above, the resin composition for the thermoplastic resinlayer 18 is dissolved in a solvent, such as alcohol, to provide acoating liquid whose viscosity is suitably controlled, and the coatingliquid is applied to the outer surface of the rubber layer 16 by a knowncoating method, such as dipping. The thus formed semi-conductivethermoplastic resin layer 18 is formed with a considerably smallthickness, without causing the problem of sagging of the coating liquid,for example, during the coating operation.

Referring next to FIG. 5 showing another embodiment of the presentinvention, a conductive roll 30 includes a conductive shaft 32, asemi-conductive rubber foam layer 34 formed on the outer circumferentialsurface of the shaft 32, a conductive solid rubber layer 36, andsemi-conductive solid rubber layer 38.

The conductive shaft 32 of the conductive roll 30 is formed of amaterial similar to that of the conductive shaft 12 of theabove-described conductive roll 10. The semi-conductive rubber layer 38is formed of a material similar to that of the semi-conductive rubberlayer 16 of the roll 10.

The semi-conductive rubber foam layer 34 formed on the outer surface ofthe conductive shaft 32 is formed of a semi-conductive rubbercomposition which contains the foaming agent as indicated above, and anion-conductive material which makes the foam layer 34 ion-conductive.The content of the ion-conductive material is desirably determined suchthat the foam layer 34 has a volume resistivity of 10⁶ -10⁹ Ωcm. In thiscase, the semi-conductive rubber foam layer 34 containing theion-conductive material provides a uniform electrical resistance, anddoes not suffer from an increase in the hardness thereof, which increasemay be induced by use of conductive particles.

The conductive solid rubber layer 36 formed on the outer surface of thesemi-conductive rubber foam layer 34 is formed of a conductive rubbercomposition that is obtained by removing the foaming agent from thematerial of which the conductive rubber foam layer 14 of the conductiveroll 10 of the previous embodiment is formed. The content of theconductive particles in the conductive rubber layer 36 is desirablycontrolled such that the rubber layer 36 has a volume resistivity of notgreater than 10⁶ Ωcm, so as to assure high uniformity in its electricalcharacteristics.

In this conductive roll 30, the semi-conductive rubber layer 38functions as a resistance adjusting layer, and the semi-conductiverubber foam layer 34 and the conductive rubber layer 36 cooperate witheach other to function as a base layer.

The thus constructed conductive roll 30 offers the same effects asprovided by the above-described conductive roll 10.

Preferably, a semi-conductive thermoplastic resin layer 40 which servesas a protective layer is formed by coating on the outer circumferentialsurface of the semi-conductive rubber layer 38 of the conductive roll30, as shown in FIG. 5. This semi-conductive thermoplastic resin layer40 is formed of a material similar to that of the semi-conductivethermoplastic resin layer 18 of the conductive roll 10 of the firstembodiment. This thermoplastic resin layer 40 advantageously reduces thestickiness or tackiness of the surface of the conductive roll 30, andeffectively prevents components of the semi-conductive rubber layer 38as the resistance adjusting layer from transferring to the roll surface.

The thus constructed conductive roll 30 is produced in the same manneras the conductive roll 10, except that the laminar structure consistingof three rubber layers 34, 36, 38 is formed on the outer surface of theconductive shaft 32.

Initially, a cylindrical laminar structure is formed by using theabove-described three kinds of rubber compositions for thesemi-conductive rubber foam layer 34, conductive rubber layer 36 andsemi-conductive rubber layer 38. This laminar structure has anunvulcanized, unfoamed rubber layer as an inner layer which gives thesemi-conductive rubber foam layer 34, and an unvulcanized, non-foamingconductive rubber layer as an intermediate layer which gives theconductive rubber layer 36, and an unvulcanized, non-foaming rubberlayer as an outer layer which gives the semi-conductive rubber layer 38.Then, the thus obtained laminar structure is placed in position within amolding cavity of a cylindrical metallic mold, with the conductive shaft32 inserted in an inner bore of the laminar structure. Thereafter, thelaminar structure is subjected to simultaneous vulcanizing and formingoperations by heating, for example, so as to provide the intendedconductive roll 30. In this embodiment, too, the unvulcanized, unfoamedlaminar structure consisting of the above three layers has an insidediameter which is larger than the outside diameter of the conductiveshaft 32, and an outside diameter which is smaller than the insidediameter of the cavity of the cylindrical metal mold. It is alsopreferable to form the laminar structure by extruding theabove-indicated three rubber layers at the same time.

The above-described method of producing the conductive roll 30 havingthe semi-conductive rubber foam layer 34, conductive rubber layer 36,and semi-conductive rubber layer 38 formed in this order on theconductive shaft 32 does not include any coating operation which uses asolvent, and therefore yield the same effects as obtained by the methodof producing the conductive roll 10.

EXAMPLES

To further clarify the principle of the present invention, there will bedescribed in detail some examples of the conductive roll constructedaccording to the present invention. However, it is to be understood thatthe present invention is by no means limited to the details of theseexamples, but may be embodied with various changes, modifications andimprovements which may occur to those skilled in the art, withoutdeparting from the scope of the invention.

EXAMPLE 1

Initially, a conductive rubber composition for forming a conductiverubber foam layer was prepared by adding 22 parts by weight of carbonblack as conductive particles to 100 parts by weight ofethylene-propylene rubber, and further adding 15 parts by weight ofdinitroso pentamethylene tetramine as a foaming agent and suitableamounts of additives, such as a vulcanizing agent.

On the other hand, a semi-conductive rubber composition for forming asemi-conductive solid rubber layer was prepared by adding 0.5 parts byweight of trimethyloctadecyl ammonium perchlorate as quaternary ammoniumsalt which serves as an ion-conductive material to 100 parts by weightof epichlorohydrin ethylene-oxide rubber, and further adding suitableamounts of additives, such as a vulcanizing agent.

Then, a coating liquid for forming a semi-conductive thermoplastic resinlayer was prepared by dispersing conductive stannic oxide in a solutionin which 8-nylon was dissolved in a suitable amount of methanol, suchthat the resultant coating liquid contained 65 parts by weight of thestannic oxide per 100 parts by weight of 8-nylon.

Suitable amounts of the thus prepared conductive rubber composition andsemi-conductive rubber composition were passed through respectiveextruders, to form by extrusion a tube which consists solely of anunvulcanized, unfoamed rubber layer 20 having an inside diameter of 8 mmand an outside diameter of 10 mm, and then form by extrusion a 0.3mm-thickness, unvulcanized, non-foaming rubber layer 22 to cover theouter circumferential surface of the tube, as shown in FIG. 2. Thus,there is obtained a continuous laminar tube 23 which has an inner layerin the form of the unvulcanized, unfoamed rubber layer 20, and an outerlayer in the form of the unvulcanized, non-foaming rubber layer 22.Thereafter, the laminar tube 23 was cut into segments each having asuitable length, to provide a plurality of laminar bodies 24.

Subsequently, an electrically conductive shaft 12 was inserted into aninner bore of each of the thus obtained laminar bodies 24, such that theshaft 12 was disposed coaxially or concentrically with the laminar body24. The conductive shaft 12 was made of iron and plated with nickel, hadan outside diameter of 6 mm, and was coated at its surface with anelectrically conductive adhesive. The laminar body 24 with theconductive shaft 12 inserted therein was then positioned within amolding cavity of a cylindricallic metal mold 26 having an insidediameter of 12 mm. Then, the laminar body 24 was heated to besimultaneously vulcanized and foamed, so as to provide a conductive rollin which the conductive rubber foam layer 14 having a volume resistivityof 10⁵ Ωcm, and the semi-conductive rubber layer 16 having a volumeresistivity of 10⁸ Ωcm were formed integrally on the outercircumferential surface of the conductive shaft 12.

After the thus obtained conductive roll was taken out of the metallicmold 26, the roll was once immersed in the coating liquid prepared inthe above-described manner, lifted up at a fixed speed, and then dried,so that a semi-conductive thermoplastic resin layer 18 was formed on theouter surface of the semi-conductive rubber layer 16. The thus formedsemi-conductive thermoplastic resin layer 18 had a thickness of 10 μm,and a volume resistivity of 10⁹ Ωcm. The hardness of this conductiveroll, when measured according to a known method, was 49 degrees (Hs: JISA).

EXAMPLE 2

A conductive roll was obtained as EXAMPLE 2 by forming the laminar body24 in the same manner as in EXAMPLE 1, except that the two rubber layers20, 22 were simultaneously extruded as shown in FIG. 3. The hardness ofthis conductive roll was 49 degrees (Hs: JIS A).

EXAMPLE 3

A conductive rubber composition for forming a conductive rubber foamlayer was prepared by adding 25 parts by weight of carbon black asconductive particles to 100 parts by weight of styrene-butadiene rubber,and further adding 18 parts by weight of azodicarbonamide as a foamingagent and suitable amounts of various additives, such as a vulcanizingagent.

Then, a semi-conductive rubber composition for forming a semi-conductivesolid rubber layer was prepared by adding 1.0 part by weight of lithiumperchlorate as structural charge specific anion which serves as anion-conductive material to 100 parts by weight of nitrile rubber, andfurther adding suitable amounts of various additives, such as avulcanizing agent.

Subsequently, a coating liquid for forming a semi-conductivethermoplastic resin layer was prepared by adding a crosslinking agentcontaining isocyanate to a solution in which urethane was dissolved in asuitable amount of methyl ethyl ketone, and further dispersingconductive titanium oxide in the solution, such that the obtainedcoating liquid contained 10 parts by weight of the crosslinking agentand 90 parts by weight of titanium oxide per 100 parts by weight ofurethane.

A conductive roll as EXAMPLE 3 was obtained in the same manner as inEXAMPLE 2 by using the thus prepared conductive rubber composition,semi-conductive rubber composition and coating liquid, as well as theconductive shaft made of the same material as used in EXAMPLE 1. In thisconductive roll, the conductive rubber foam layer, semi-conductiverubber layer and semi-conductive thermoplastic layer had volumeresistivities of 10⁵ Ωcm, 10⁸ Ωcm and 10⁹ Ωcm, respectively. Thehardness of the conductive roll was 47 degrees (Hs: JIS A).

EXAMPLE 4

A semi-conductive rubber composition was prepared by adding 30 parts byweight of carbon block as conductive particles and suitable amounts ofvarious additives, such as a vulcanizing agent, to 100 parts by weightof urethane rubber. A conductive roll as EXAMPLE 4 was obtained in thesame manner as in. EXAMPLE 2, except that the semi-conductive solidrubber layer was formed of the thus prepared semi-conductive rubbercomposition. The hardness of the obtained conductive roll was 49 degrees(Hs: JIS A).

EXAMPLE 5

Initially, a semi-conductive rubber composition for forming asemi-conductive rubber foam layer was prepared by adding 2 parts byweight of trimethyloctadecyl ammonium perchlorate as quaternary ammoniumsalt which serves as an ion-conductive material, to 100 parts by weightof epichlorohydrin ethylene-oxide rubber, and further adding 10 parts byweight of dinitroso pentamethylene tetramine as a foaming agent, andsuitable amounts of additives, such as a vulcanizing agent.

Then, a conductive rubber composition for forming a conductive solidrubber layer was prepared by adding 20 parts by weight of carbon blackas conductive particles to 100 parts by weight of ethylene-propylenerubber, and further adding suitable amounts of various additives, suchas a vulcanizing agent.

Subsequently, a semi-conductive rubber composition for forming asemi-conductive solid rubber layer was prepared by adding 0.2 parts byweight of trimethyloctadecyl ammonium perchlorate as quaternary ammoniumsalt which serves as an ion-conductive material, to 100 parts by weightof epichlorohydrin ethylene-oxide rubber, and further adding suitableamounts of additives, such as a vulcanizing agent.

Then, a coating liquid for forming a semi-conductive thermoplastic resinlayer was prepared by dispersing conductive stannic oxide in a solutionin which 8-nylon was dissolved in a suitable amount of methanol, suchthat the coating liquid contained 65 parts by weight of stannic oxideper 100 parts by weight of 8-nylon.

In the next step, the above-described three kinds of rubber compositionswere simultaneously passed through a suitable extruder, so as to form acontinuous laminar tube which has an inner layer in the form of anunvulcanized, unfoamed rubber layer which will be foamed to provide thesemi-conductive rubber foam layer, an intermediate layer in the form ofan unvulcanized, non-foaming conductive rubber layer which gives theconductive rubber layer, and an outer layer in the form of anunvulcanized, non-foaming semi-conductive rubber layer which gives thesemi-conductive rubber layer. The inner layer of the laminar tube formedby simultaneous extrusion had an inside diameter of 8 mm and an outsidediameter of 10 mm, and the intermediate layer and the outer layer hadrespective thickness values of 0.1 mm and 0.3 mm. Thereafter, thelaminar tube was cut into a plurality of laminar bodies each having athree-layered structure.

Then, each of the thus obtained laminar bodies was positioned withrespect to the conductive shaft and cylindrical metal mold as used inEXAMPLE 1, and vulcanized and foamed, so as to provide a conductive rollin which the semi-conductive rubber foam layer, conductive rubber layer,and semi-conductive rubber layer were laminated in this order on theouter circumferential surface of the conductive shaft. Thesesemi-conductive rubber foam layer, conductive rubber layer andsemi-conductive rubber layer had volume resistivity of 10⁸ Ωcm, 10⁴ Ωcmand 10⁸ Ωcm, respectively.

The thus obtained conductive roll was then subjected to the coatingoperation as effected in EXAMPLE 1, using the coating liquid prepared asdescribed above, so that the semi-conductive thermoplastic resin layerwas formed on the outer surface of the semi-conductive rubber layer.This thermoplastic resin layer had a thickness of 10 μm and a volumeresistivity of 10⁹ Ωcm. The hardness of this conductive roll was 41degree (Hs: JIS A).

COMPARATIVE EXAMPLE 1

For comparison, a conductive roll was produced by a conventional coatingmethod in the following manner. Initially, a conductive rubbercomposition for forming a conductive solid rubber layer as a base layerwas prepared by adding 25 parts by weight of carbon black as conductiveparticles to 100 parts by weight of styrene-butadiene rubber, andfurther adding suitable amounts of additives, such as a vulcanizingagent.

Then, a coating liquid for forming a softener-preventing layer wasprepared by adding water-soluble melamine as a crosslinking agent to asolution in which 8-nylon was dissolved in a suitable amount ofmethanol, and dispersing carbon black in the solution, such that thecoating liquid contained 30 parts by weight of the water-solublemelamine and 12 parts by weight of the carbon black per 100 parts byweight of 8-nylon.

Subsequently, a coating liquid for forming a resistance adjusting layerwas prepared by adding trimethyloctadecyl ammonium perchlorate to asolution in which epichlorohydrin ethylene-oxide rubber is dissolved ina suitable amount of methyl ethyl ketone, such that thetrimethyloctadecyl ammonium perchlorate amounted to 0.2 parts by weightper 100 parts by weight of the epichlorohydrin ethylene-oxide rubber.

Then, a coating liquid for forming a protective layer was prepared bydispersing conductive stannic oxide in a solution in which 8-nylon wasdissolved in a suitable amount of methanol, such that the coating liquidcontained 65 parts by weight of conductive stannic oxide per 100 partsby weight of 8-nylon.

In the next step, a conductive shaft having an outside diameter of 6 mmwas positioned at the center of a molding cavity of a cylindricalmetallic mold having an inside diameter of 12 mm. The conductive shaftwas made of iron and plated with nickel, and was coated at its surfacewith a suitable conductive adhesive. Then, the cavity of the cylindricalmold was filled with the conductive rubber composition prepared asdescribed above. This rubber composition was then heated and vulcanized,so as to provide a base roll in which the conductive rubber layer wasformed integrally on the outer surface of the conductive shaft.

Subsequently, the thus obtained base roll was taken out of thecylindrical mold, dipped or immersed for a while in the coating liquidfor the softener-preventing layer, and lifted up at a fixed speed. Afterdrying by air, the coating liquid was heated so as to form thesoftener-preventing layer having a thickness of 10 μm and a volumeresistivity of 1×10⁴ Ωcm on the outer circumferential surface of theconductive rubber layer as the base layer.

Then, the base roll with the softener-preventing layer formed thereonwas subjected to three coating cycles using the previously preparedcoating liquid for the resistance adjusting layer, each cycle includingdipping, lifting-up and drying steps as described above. Thereafter, thecoating layer was cross-linked by heating so as to form the resistanceadjusting layer having a thickness of 160 μm and a volume resistivity of3×10⁸ Ωcm on the outer circumferential surface of thesoftener-preventing layer.

Finally, the base roll which bears the softener-preventing layer andresistance-adjusting layer thereon was coated with the previouslyprepared coating liquid for the protective layer, in the manner asdescribed above, so that the protective layer having a thickness of 10μm and a volume resistivity of 5×10⁹ Ωcm was formed on the outercircumferential surface of the resistance adjusting layer. In thismanner, a conductive roll as COMPARATIVE EXAMPLE 1 was obtained. Thehardness of the conductive roll was 62 degree (Hs: JIS A).

COMPARATIVE EXAMPLE 2

A conductive roll as COMPARATIVE EXAMPLE 2 was produced in the followingmanner. Initially, a cylindrical rubber tube was formed by extrusionusing the semi-conductive rubber composition for the semi-conductiverubber foam layer as used in EXAMPLE 5, and then cut into a plurality ofunvulcanized, unfoamed rubber bodies each having a suitable length. Thethus obtained rubber body which gives a semi-conductive rubber foamlayer had an inside diameter of 8 mm and an outside diameter of 11 mm.Then, the conductive shaft as used in COMPARATIVE EXAMPLE 1 was insertedinto the unvulcanized, unfoamed rubber body, which was in turn insertedinto a thermoplastic polyelastomer tube which was formed by extrusionwith an outside diameter of 11.5 mm and a thickness of 0.2 mm and whichgives a conductive thermoplastic elastomer layer. Thus, a laminarstructure having the shaft, rubber body and polyelastomer tube wasobtained. This laminar structure was then positioned in the cavity ofthe cylindrical metallic mold as used in COMPARATIVE EXAMPLE 1, andvulcanized and foamed by heating, to provide a base roll which had abase layer consisting of the semi-conductive rubber foam layer andconductive thermoplastic elastomer layer, and the conductive shaftbearing the base roll thereon.

Then, a softener-preventing layer, resistance adjusting layer andprotective layer were formed in this order on the outer circumferentialsurface of the base roll constructed as described above, by coating thebase roll with the coating liquids for forming these layers as used inCOMPARATIVE EXAMPLE 1. In this manner, the conductive roll having ahardness of 48 degrees (Hs: JIS A) was obtained.

The thus obtained conductive rolls of EXAMPLES 1 through 5 andCOMPARATIVE EXAMPLES 1 through 2 were evaluated in respect of variouscharacteristics in the manners as described below. The results of theevaluation are indicated in TABLE 1 below.

Contact with Photosensitive Drum

Each of the above conductive rolls was brought into contact with asmooth, metallic roll having the same outside diameter (30 mm) as thephotosensitive drum, with a load of 500 g being applied to one end ofthe conductive roll. Then, the metallic roll was rotated with a lightsource located on one side of the metallic roll and the conductive roll.A nip formed between the metallic roll and the conductive roll wasobserved from the side opposite to the above-indicated one side, tocheck if a light from the light source came through a clearance formedbetween these two rolls. In TABLE 1, O indicates that the light was notseen while indicates that the light was slightly observed through theclearance.

Leak Voltage

The conductive roll was brought into contact with a smooth, metallicroll having the same outside diameter (30 mm) as the photosensitivedrum, with a load of 500 g being applied to one end of the conductiveroll. In this condition, a DC voltage was applied to the conductiveroll, and a voltage at which discharge breakdown took place, that is, aleak voltage, was measured.

Chargeable Voltage

The conductive roll serving as a charging roll was actually installed ina printer ("Laser Jet 4 Printer" manufactured by HEWLETT PACKARD), andAC voltages were measured at respective times when fog disappeared andwhen a defective image caused by abnormal discharge appeared, with avoltage of XVp-p500 Hz-600 V being applied by an external power source.

Noise

The conductive roll serving as a charging roll was actually installed ina printer ("Laser Jet 4 Printer" manufactured by HEWLETT PACKARD) fromwhich a member for preventing noise of a photosensitive drum wasremoved. The printer was operated in this condition, to check if thenoise was made or not. In TABLE 1, O indicates that no noise occurred,and X indicates that the noise was made.

It will be apparent from TABLE 1 that the conductive rolls of EXAMPLES 1through 5 were held in contact with the metallic roll with no clearancetherebetween, and exhibited apparently higher lead voltage and abnormaldischarge voltage than the conductive rolls of COMPARATIVE EXAMPLES 1and 2. Further, the conductive rolls of EXAMPLES 1-5 made no noise. Thismeans that the conductive roll constructed according to the presentinvention has a good contact with the photosensitive drum, and highresistance to leak (leakage of electric current), and is alsoadvantageously free from abnormal discharge and noise.

                                      TABLE 1                                     __________________________________________________________________________                                             COM-                                                                          PARATIVE                                        EXAMPLES                      EXAMPLES                                        1     2     3     4     5     1   2                                __________________________________________________________________________    Contact with                                                                             ∘                                                                       ∘                                                                       ∘                                                                       ∘                                                                       ∘                                                                       Δ                                                                           Δ                          Photosensitive Drum                                                           Leak Voltage (V)                                                                         4.0<  4.0<  4.0<  4.0<  4.0<  3.0 3.1                              Chargeable                                                                    Voltage                                                                       Fog Disappearing                                                                         1600  1600  1600  1600  1700  1600                                                                              1700                             Voltage (V)                                                                   Abnormal Discharge                                                                       4000< 4000< 4000< 4000< 4000< 2700                                                                              3500                             Voltage (V)                                                                   Noise      ∘                                                                       ∘                                                                       ∘                                                                       ∘                                                                       ∘                                                                       X   ∘                    __________________________________________________________________________

What is claimed is:
 1. A conductive roll comprising:a conductive shaft;a rubber foam layer formed on an outer circumferential surface of saidconductive shaft; and a semi-conductive solid rubber layer providedradially outwardly of said rubber foam layer.
 2. A conductive rollaccording to claim 1, wherein said rubber foam layer consists of aconductive rubber foam layer, and said semi-conductive solid rubberlayer is formed on an outer circumferential surface of said conductiverubber foam layer.
 3. A conductive roll according to claim 1, whereinsaid rubber foam layer consists of a semi-conductive rubber foam layer.4. A conductive roll according to claim 3, further comprising aconductive solid rubber layer interposed between said semi-conductiverubber foam layer and said semi-conductive solid rubber layer.
 5. Aconductive roll comprising:a conductive shaft; a conductive rubber foamlayer formed on an outer circumferential surface of said conductiveshaft; and a semi-conductive solid rubber layer formed on an outercircumferential surface of said conductive rubber foam layer.
 6. Aconductive roll according to claim 5, further comprising asemi-conductive thermoplastic resin layer formed by coating on an outercircumferential surface of said semi-conductive solid rubber layer, saidsemi-conductive thermoplastic resin layer being made principally of acomposition which contains one of a thermoplastic resin containingconductive particles dispersed therein, and said thermoplastic resinfurther containing a crosslinking agent.
 7. A conductive roll accordingto claim 5, wherein said conductive rubber foam layer containsconductive particles, and has a volume resistivity of not greater than10⁶ Ωcm.
 8. A conductive roll according to claim 7, wherein saidconductive particles comprise one of carbon black, graphite, a metalpowder and a conductive metal oxide.
 9. A conductive roll according toclaim 5, wherein said semi-conductive solid rubber layer contains anion-conductive material, and a volume resistivity of 10⁶ Ωcm -10¹⁰ Ωcm.10. A conductive roll according to claim 9, wherein said ion-conductivematerial is selected from the group consisting of quaternary ammoniumsalts including trimethyloctadecyl ammonium perchlorate andbenzyltrimethyl ammonium chloride, and structural charge specific anionsincluding lithium perchlorate and potassium perchlorate.
 11. Aconductive roll comprising:a conductive shaft; a semi-conductive rubberfoam layer formed on an outer circumferential surface of said conductiveshaft; a conductive solid rubber layer formed on an outercircumferential surface of said semi-conductive rubber foam layer; and asemi-conductive solid rubber layer formed on an outer circumferentialsurface of said conductive solid rubber layer.
 12. A conductive rollaccording to claim 11, further comprising a semi-conductivethermoplastic resin layer formed by coating on an outer circumferentialsurface of said semi-conductive solid rubber layer, said semi-conductivethermoplastic resin layer being made principally of a composition whichcontains one of a thermoplastic resin containing conductive particlesdispersed therein, and said thermoplastic resin further containing acrosslinking agent.
 13. A conductive roll according to claim 11, whereinsaid semi-conductive rubber foam layer contains an ion-conductivematerial, and a volume resistivity of 10⁶ Ωcm-10⁹ Ωcm.
 14. A conductiveroll according to claim 11, wherein said conductive solid rubber layercontains conductive particles, and has a volume resistivity of notgreater than 10⁶ Ωcm.
 15. A conductive roll according to claim 11,wherein said semi-conductive solid rubber layer contains anion-conductive material, and a volume resistivity of 10⁶ Ωcm -10¹⁰ Ωcm.